nixredo/lib.nix

{ lib ? import <nixpkgs/lib> }:
let
  inherit (builtins) baseNameOf dirOf length genList pathExists;
  inherit (lib) take splitString concatStringsSep last foldl foldr head
  tail singleton removePrefix hasSuffix removeSuffix flatten crossLists reverseList;
  butlast = list: take (length list - 1) list;

  doFileSuffix = "do";

  genPatterns = path: let
    fn = baseNameOf path;
  in (map (x: "default." + x)
       (foldr (a: b: let x = (head b); in [ (a + "." + x) ] ++ b)
         [ doFileSuffix ]
         (tail (splitString "." fn))));

  searchPath = path:
    let
      pathlist = (splitString "/" path);
      pat = (last pathlist);
      doFile = path + "." + doFileSuffix;
      patlist = genPatterns pat;
      foldedpaths = foldr (a: b:
        let
          h = if length b == 0 then "" else head b;
        in [ (h + a + "/") ] ++ b) [] (tail (reverseList pathlist));
      pths = crossLists (p: pat: p + pat) [ foldedpaths patlist ];
    in [ doFile ] ++ pths;

  cwd = "/home/spacefrogg/tmp";
  path = cwd + "/foo/bar/baz.def.c";

  whichdo = path: let
    pathlist = searchPath path;
  in foldl (res: p: if res == "" then if pathExists p then p else res else res) "" pathlist;

  d1 = path: pat: let
    dir = (dirOf pat) + "/";
  in removePrefix dir path;

  d2 = path: pat: let
    dir = (dirOf pat) + "/";
    _path = removePrefix dir path;
    _pat = removeSuffix ".${doFileSuffix}" (removePrefix "${dir}default" pat);
    _out = removeSuffix _pat _path;
  in if _out == "" then _path else _out;

  self = {
    inherit searchPath cwd path doFileSuffix whichdo d1 d2;
  };

in self